An apical disc in which the ocular and genital plates form a single contiguous area not surrounding the periproct.
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compass

Part of the Aristotle's lantern; a Y-shaped ossicle (c) that overlies the rotula. Its function is to control the volume of the pharyngeal coelom as the lantern moves in and out of the test.
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complemental plates

Small plates from the periproct that are incorporated into the apical disc in irregular echinoids after the anus has moved posteriorly out of the apical disc.
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complex plate compounding

A style of ambulacral plate compounding in which subgroups of three or more elements become united together to form a more complex repeat pattern
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compound (plate)

An ambulacral plate that is composed of two or more individual elements. Various patterns of compound plating are distinguished and provide diagnostic taxonomic characters. These include diadematid, phymosomatid, arbaciid, glypticid and echinid compounding.
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conjugate (pore-pair)

A pore-pair where the two pores are connected by a shallow furrow or groove. These are always associated with specialized respiratory tube-feet.
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corona

The calcitic skeleton of plates excluding the apical and peristomial plates.
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cortex

A superficial layer of calcite deposited over the shaft of a spine. This layer is one of the few parts of the echinoid test that is not flesh-covered.
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crenulate (tubercle)

A tubercle with a toothed or noded platform surrounding the mamelon.

crenulation (tubercle)

The ring of cog-like projections on a tubercle. These interlock with similar indentations around the base of the spine for securer articulation.
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